Hi, i already reinstall antiX with xfce in spanish again, with another iso. I used a very simple root passwd of just tree letters for this spanish installation.

Now i need to change to a strong passwd, last time i tried to do that, i was not able to loging again into the system.

How can i change to a strong passwd without to make any mistake? -- right now my system is in spanish but there is *something* that still been in english. This is a bit complicated to me, i'm going to need some help with this last problem. Thanks.

you did this?
$su
(old root password)
#passwd root
(follow the instructions, that is, enter the new root password twice)

IIRC When you execute things as root using sudo you have to type the password of the user instead.
When you do this and remaster it should stick when you use "personal" settings and revert to the 'root' 'demo' defaults if you use "general". Iso building I don't recall, but might be the same. No idea for persistence, while in a plain HD installation you don't need to do anything. Maybe mastering did reset to the simple password so you could not login with the stronger one.

The user manager application provides a UI for accomplishing various tasks, including set/change password.
For "safety" the application requires you to type the password twice. It checks whether these match and, if not, will warn "First and second password must be identical".

a command to see clear

I don't recall (and can't check right this moment) whether the text entry boxes show the typed characters in "clear" plain text, or as asterisk placeholders.